env: Rename getenv_hex(), getenv_yesno(), getenv_ulong()

We are now using an env_ prefix for environment functions. Rename these
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/source.c

@@ -40,7 +40,7 @@ source (ulong addr, const char *fit_uname)
 	size_t		fit_len;
 #endif
 
-	verify = getenv_yesno ("verify");
+	verify = env_get_yesno("verify");
 
 	buf = map_sysmem(addr, 0);
 	switch (genimg_get_format(buf)) {